Atherosclerosis, narrowing of the blood vessels, is a common disease of advancing age. When important vessels narrow beyond a certain limit, damage to major organs of the human body may occur, for example, heart attacks and strokes. In addition, organs that have a chronically impaired blood supply do not function properly, leading in many cases to debilitating illnesses.
One of the most common treatments for narrowed blood vessels is angioplasty. During an angioplasty procedure, a balloon is inserted into the narrowed portion of the blood vessel and inflated, thereby widening the lumen. In a supplementary technique, a stent is inserted into the blood vessel at the widened location to support the vessel (which may have been damaged by the angioplasty) and to maintain an open lumen. In extreme cases a portion of the circulatory system may be replaced with a tissue graft or with a graft manufactured of biocompatible materials. Newer stents are radio-opaque, and are visible under fluoroscopy, which aids in guiding them into position.
It frequently occurs, however, that the blood vessel narrows again after the procedure, either by accumulation of plaque or other material inside the stent or graft, or by (unrelated) narrowing of the blood vessel at other locations. Thus, in many cases, an angioplasty procedure must be repeated after a time, or bypass surgery must be performed on the patient. It would be desirable to monitor the flow of blood inside the stent or graft, so as to determine whether significant narrowing has occurred.
Various methods of measuring flow velocities and volumes are known in the art, including ultrasonic and electromagnetic sensors. For example, U.S. Pat. No. 5,522,394, the disclosure of which is incorporated herein by reference, describes an implantable probe for measuring blood flow velocity. U.S. Pat. No. 5,205,292, the disclosure of which is incorporated herein by reference, describes an implantable probe for measuring blood flow velocity and other physiological parameters, which is attached to the outside of a blood vessel. U.S. Pat. No. 4,109,644, the disclosure of which is incorporated herein by reference, describes an implantable wireless ultrasound probe which is powered by external electromagnetic induction and transmits outside the body, via electromagnetic radiation, a signal indicative of ultrasound signals received by the probe.
In Doppler ultrasound, the flow is irradiated with ultrasonic waves at a given frequency. The reflection of the ultrasound from the flow is shifted by the Doppler effect to a different frequency from the transmitted frequency. Since the Doppler shift is linearly related to the flow velocity, the velocity can be determined by analyzing the frequency spectrum of the reflection.
Another type of ultrasound flow meter is the transit time flow meters, which takes advantage of a difference in propagation velocity between upstream propagation and downstream propagation of ultrasound waves. Waves traveling in the direction of flow move faster than waves which travel in a direction opposite the flow.
Electromagnetic flow meters (EMF) use the well-known property that a voltage potential develops across a conductor moving in a magnetic field. To generate an EMF in a blood vessel, it is surrounded by an induction coil, which generates a magnetic field on the blood vessel. The voltage that develops on the blood is measured on the blood vessel rather than on the blood.
Miniature transmitters, suitable for implantation in the human body for transmission of physiological parameters are well known in the art.
"Bio-Medical Telemetry (second edition)", by R. S. Mackay, published by the IEEE press, 1993, the disclosure of which is incorporated herein by reference, describes several types of miniature transmitters, which transmit measurements from implanted physiological sensors to receivers outside the body. In particular, Mackay, in chapter 5, pp. 111-147 describes pressure sensors for measuring various physiological parameters, including blood pressure. On page 143, Mackay describes a variable inductance pressure sensor which is attached to the outside of a vessel. The sensor includes a transmitter, which includes a coil coupled to the pressure sensor, such that a ferrite moves axially within the coil in response to changes in pressure, so as to cause a variation in the inductance of the coil and frequency-modulate the radiation that the coil transmits. such a transmitter is also described in U.S. Pat. No. 5,497,147, the disclosure of which is incorporated herein by reference.
On page 138, Mackay suggests placing a pressure sensor into the vascular system, such as into an artificial heart valve, but recommends against placing a pressure sensor inside the blood stream due to the obstruction caused by the pressure sensor and by the danger of forming blood clots. It is however known to place pressure sensors into the blood stream to directly measure pressure, for example, in a study described on page 331 of Mackay in which a pressure sensor was inserted into an aorta of a baboon. It is also known to use intra-vascular pressure sensors to control demand responsive pacemakers.
Chapter 10 of Mackay, pp. 298-315, describes various methods of passive transmission, wherein the energy for sensing and transmission is provided by an outside source. In particular, a transmitter using a tunnel diode, two capacitors and a coil is described, in which one capacitor stores energy during a charging phase and releases the energy to an oscillator which includes the other capacitor, the coil and the tunnel diode. The transmitted frequency is offset from the received frequency.
Use of flow of blood in a magnetic field to generate power for an implantable device, is described on page 70 of Mackay, and is suggested as being suitable only in large blood vessels. Using the voltage generated by such a flow for an implantable flowmeter is not suggested by Mackay.
U.S. Pat. No. 4,656,463, the disclosure of which is incorporated herein by reference, describes a LIMIS (Location Identification, measuring of Movement of Inventory System) which uses miniature coded transponders.
U.S. Pat. Nos. 5,073,781, 5,027,107 and 5,293,399, the disclosures of which are incorporated herein by reference, describe miniature transponders having no internal power supply.
U.S. Pat. No. 5,483,826, the disclosure of which is incorporated herein by reference, describes a miniature transponder for transmitting pressure values of a vessel, such as a tire.
U.S. Pat. No. 5,105,829, the disclosure of which is incorporated herein by reference, describes a passive transmitter which is adapted for implantation into the human body and which is capacitively coupled to the body to better transmit a signal indicative of a foreign object left in the body during a surgical procedure.